A 5 cell bulb will work if you have let batteries rest a little while and the combo has settled down to 7.4 volts.
But lithium ion batteries don't settle. If they go below 4.2v after charging, they're getting bad.
My Nitecore RCR's come off the charger at 4.2 to 4.3. After setting over night they are at 3.7 to 3.8
Lithium ion battery should go no less than 2.5 volt and no more than 4.25. After fully recharge, they are about 4.2 volt.
If a fully recharged cell after resting has 3.7 volt which is about 25% energy left, then it's time to replace or something is wrong.
A good and fully recharged Lithium ion battery should go no less than 4 volt after sitting a month.
A 6 volt (5 cell) Xenon bulb with life of 40 hours will flash with regulated and fixed 7.4 volt. Let alone 8 or 8.4 volts.
If anybody drive this setup with no problem, there is definitely some voltage drop on batteries, switch, connections and ....